Polymer of dithioether-linked phthalonitrile

ABSTRACT

Dithioether-linked phthalonitrile monomer are prepared by a substitution reaction between 4-nitrophthalonitrile and a dimercaptan and are polymerized to a high-temperature, oxidation-resistant polymer by heating them at a temperature above their melting point. The rate of polymerization is increased by the addition of an amine. Electrical conductivity of the polymer can be increased to the conductor range by heating the polymer above about 400° C.

BACKGROUND OF THE INVENTION

This invention relates generally to the synthesis and polymerization ofphthalonitrile monomers, and, more particularly to the synthesis,polymerization and pyrolization of dithioether-linked phthalonitrilemonomers.

Phthalonitrile monomers have two phenyl dinitrile groups connected by alinking group. The linking group in the monomer becomes the linkinggroup in the polymer. The properties of the polymer are determined to alarge degree by the linking group. Heretofore, the linking group hasbeen generally hydrocarbons, phenoxy, alkoxy or sulfone moieties.

Examples of previous phthalonitrile monomers and polymers demonstratethe versatility and importance of this growing class of materials.Keller, in the U.S. Pat. No. 4,234,712, used sulphone and ketone groupsto link phthalocyanines, thus forming high temperature structuralcomposites and adhesives. Similarly, Keller, in U.S. Pat. No. 4,238,601,used perfluorinated alkyl substituents on the linking and phenyl groupsto increase the corrosive resistance of polymerized phthalocyanines.Polyphenylether-bridged polyphthalocyanines were prepared by Keller,U.S. Pat. No. 4,259,471, which exhibited exceptional thermal stabilityand oxidative resistance. Also, by varying the fluoride content of theconstituent and substituent groups of the linking group, Keller, in U.S.Pat. No. 4,315,093, produced a phthalonitrile polymer with improvedwater repellancy and resistance to oxidative attacks. The performance ofthe polymers have exceeded polyoxides, epoxies and other similarpolymers in many applications.

One property of high-temperature organic materials that is of greatinterest is electrical conductivity. Materials with high conductivitieshave been obtained by either chemical doping of a linear conjugatedsystem or by thermal treatment of polymers. In the case of chemicaldoping, the conductivity of a polymer is varied and controlled by theamount of dopant used. For thermal treatment, the conductivity of apolymer is changed by a precise pyrolytic procedure. One polymer thathas produced a highly conductive organic material through thermaltreatment is the diether-linked phthalonitrile polymer of U.S. Pat. No.4,587,325. It has been determined that the ether linkages interruptelectronic delocalization. Hence a pyralyzed polymer without etherlinkages would have a better electrical stability.

SUMMARY OF THE INVENTION

It is, therefore, an object of this invention to replace metals andinorganic semiconductors and conductors with a non-doped conductiveorganic polymer with substantial structural strength and thermalstability.

Another object of this invention is to prepare this conductive polymerfrom an easily processable, low-cost phthalonitrile monomer.

Another object of this invention is to convert this polymer into ahighly conductive material by thermal means.

Yet another object of this invention is to produce a polymer with apredetermined conductivity over a wide range from insulator tosemiconductor to conductor by a simple change in the polymerizationtemperature.

A further object of this invention is to provide a conductive polymerwhich is environmentally stable.

A still further object of this invention is to provide a conductivepolymer for use at elevated temperatures in an oxidizing environment.

These and other objects of the invention are achieved by polymerizingdithioether-linked phthalonitrile monomers with a thermally stable aminecuring agent at a temperature between the melting point of the monomersand thermal decomposition point for the polymers.

DETAILED DESCRIPTION OF THE INVENTION

The phthalonitrile monomers of the present invention has the formula:##STR1## wherein A is a linear or branched, saturated or unsaturatedaliphatic hydrocarbon having from 2 to 30 carbon atoms, the preferredbeing a linear, saturated hydrocarbon having 5 to 15 carbon atoms, anaromatic or saturated, single or multiple ring cyclic hydrocarbon havingfrom 4 to 30 carbon atoms, the preferred being multiple aromatic rings,especially biphenyl; or a fused ring hydrocarbon, preferable a fusedring structure containing only aromatic units, particularly naphthaleneor anthracene. The criteria for the linking group, (-S-A-S-) is that themoiety has at least two thio-based linkages, thermally stable at thepolymerization temperatures of the monomer. Furthermore, the linkinggroup cannot contain a functional group that is reactive with thenitrile group of the 4-nitrophthalonitrile. The preparation of thephthalonitrile can be summarized by the following reaction: ##STR2## Thepreparation comprises mixing 4-nitrophthalonitrile with a dithiol and abase in a dipolar aprotic solvent.

The dithiol and 4-nitrophthalonitrile are preferably mixed in aapproximate stoichiometric amount, i.e., a dithiolnitrophthalonitrilemole ratio of 1:2. The solvent is added in an amount at least sufficientto dissolve the reactants. The preferred solvents are dimethyl sulfoxide(DMSO), N, N-dimethylformamide (DMF), N-methyl pyrrolidine (NMP) andN,N-dimethyl acetamide with DMSO being the most preferred.

The base removes a hydrogen from the thio group. At least astoichiometric amount of the base is added and preferably a 10 to 25mole percent excess is added. It is noted that excess base can betrapped in the product; so, extra care in the workup and purificationshould be used whenever an excess of base is used. The preferred basesare lithium, sodium, potassium, or calcium hydroxide; sodium, potassiumor calcium carbonate; organolithium reagents such as methyl or n-butyllithium Grignard agents and sodium or potassium bicarbonate. The mostpreferred bases are sodium or potassium hydroxide and potassiumcarbonate.

If a hydroxide-type base is used, water is formed by the reaction of thebase with the protic proton of the thio group. Since the cyano groupsreact with hydroxide in the presence of water, it is necessary to removeall water from the reaction mixture before adding thenitrophthalonitrile reactant. For this reason, the method utilizing awater-producing base requires the initial steps of mixing the solvent,base, dithiol and a refluxing liquid, e.g., benzene or toluene until allwater (as determined by the stoichiometric equation) has been removed byazeotropic distillation. The solution is then cooled to about roomtemperature before adding the nitrophthalonitrile. After thenitrophthalonitrile has been added, the reaction solution is slowlyheated to a temperature from about 20° V to about 100° C. and preferablyfrom 25° C. to 80° C. and heating is maintained until the reaction iscompleted, as determined by e.g., monitoring the disappearance of theNO₂ absorption at 1539 and 1358 cm⁻¹ in the IR spectrum. Next, thereaction solution is cooled to about room temperature and poured intocold water (about 10° C. or less). The product is collected by, e.g.,suction filtration, washed with water, and dried in an oven at reducedpressure.

If the base does not produce water, such as, a carbonate or bicarbonate,the reaction can be performed in one step. The reactants, base, andsolvent are added to a reaction vessel in one step. The reaction mixtureis then heated to a temperature from about 20° C. to about 100° C. andpreferably from 25° C. to 80° C. Upon completion the reaction mixture isslowly poured into cold dilute hydrochloric acid (2N or less) at atemperature below 15° C. The product yield can be increased if thecarbonate or bicarbonate is added in increments with no increment beinglarger than one-third of the total base. When the base is added in oneportion, the reaction may not proceed to completion, which is probablyattributed to the surface of the base becoming coated during the courseof the reaction.

The monomer synthesis is done in an inert atmosphere and preferably in anonoxidizing atmosphere. The most preferred atmospheres are argon,helium and nitrogen.

Polymerization of the subject monomers is believed to afford a complexstructural mixture similar in kind to other phthalonitrile polymers. Todate physical analysis have not been able to determine the actualstructure of the polymer.

The polymer can be synthesized neat, or in the presence of powderedmetals, metallic salts or amine curing agents. Neat polymerization isextremely difficult and slow due to the absence of active hydrogenatoms. The addition of powdered metals or metallic salts improves thereaction time but also presents other problems, e.g. voids in thepolymer. The cure time and polymerization temperature are reducedsignificantly in the presence of an amine curing agent.

The amine curing agent, when present in minute quantities, probablyattacks the nitrile groups of the dinitrile reactant, precipitating theformation of N-substituted-3 iminoisoindoline intermediates. These unitscan propagate the polymerization reaction by reacting with other nitrilegroups. As the propagating reaction progresses, other reaction pathwaysinvolving polytriazine and polyphthalocyanine formations may also bepresent.

The amine curing agents used to polymerize the subject monomers can beany monoamine or polyamine (containing more than one amine unit) thatdoes not volatilize too quickly or decompose at the polymerizationtemperatures. The preferred amine curing agents are diamines. Examplesof amine curing agents are m- and p-phenylenediamine,4-aminophenylether, 3-aminophenyl sulfone, 4-aminophenyl sulfone, 4,4'-(p-phenylenedioxy)dianiline and 4,4' methylenedianiline, and apolyamine represented by the formula: ##STR3## wherein n is 0, 1, 2, 3,4, or 5; X is a hydrogen, a halogen, a halocarbon, an alkyl, an aminogroup or a amino group substituted with alkyls; Y is a hydrogen or anamino group; Y' is a hydrogen or a amino group and at least one Y or Y'must be an amino group. The most preferred amine curing agent is theabove polyamine wherein n is 1, 2, or 3; X is a hydrogen, chlorine,bromine, an alkyl of five carbons or less an amino group or an aminogroup substituted with an alkyl of five carbons or less; Y is an aminogroup and Y' is a hydrogen. The most preferred amine curing agent is 1,3-bis(3-aminophenoxy)benzene. These polyamine curing agents aredisclosed and claimed in the U.S. Patent Application by Teddy M. Kellerentitled "Curing Agent for High-Performance Phthalonitrile Resin" andfiled on Nov. 17, 1988 which is incorporated herein by reference.Aliphatic amines can be used but they reduce the thermal and oxidativestability of the polymers. The amount of the curing agent is from about0.5 to weight percent the stoichiometric amount of the amine agent about10 percent of the monomer weight, preferably from 1 to 2 weight percent.

Polymerization proceeds above the melting point of the monomer. It ispreferred that the monomer and the amine are heated slightly above themonomers melting point until the monomer is converted to the amorphousstate. The temperature is preferably from about 10° to 80° C. above themelting point and preferably from 10° to 25° C. above the melting point.After the monomer has reached the glassy state, the glass transitiontemperature (Tg) is greatly depressed relative to the crystalline state.Accordingly, the processing temperature to complete the polymerizationcan be lowered, resulting in the ability to control the polymerizationreaction. After gelation, the polymer is preferably heated attemperature from 260° C. to 315° C. to enhanced the physical properties.The polymerization can be stopped after the polymer has reached theB-stage. At the B-stage, the polymer is a frangible solid which can beeasily pulverized. This capability permits great versatility in thefabrication of finished products during the curing and postcuringprocedures. Thermal stresses are best avoided by slowly heating andcooling the monomer and polymer. Heating can proceed rapidly up to aboutthe melting point of the monomer. After the polymerization is completed,the polymer is preferably cooled from 0.2° to 1° C./min and mostpreferably from 0.3° to 0.6° C./min. The polymerization reaction carriedout up to 315° C. can be performed in air. When postcured attemperatures in excess of 315° C., the polymer is heated in an inertatmosphere. The most preferred postcured atmospheres are argon, heliumand nitrogen.

In summary, polymerization can proceed at a temperature from the monomermelting point to its decomposition temperature. It is possible to lowerthe temperature after the monomer has become amorphous and proceed withthe polymerization reaction at a much lower temperature relative to themonomer melting point. The polymerization can be conveniently stopped atthe B-stage and be rendered into a powder or be stored indefinitely atroom temperature. Doped polymers are not preferred. It is a significantadvantage of the present invention that electrical conductivity over awide range can be achieved without doping. However, the present polymerscan be doped with, for example, a salt or a metal. Depending on theamount of the salt or powdered metal being added, all or some of thesalt or metal may comply with segments of the resulting polymers. Thecomplying mechanism is evidenced by the increase in the polymerizationrate. Whether the salt or metal actually becomes part of the polymer,the additive does reduce the oxidative and thermal stability. Thepreferred salt in that regard is stannous chloride dihydrate (SnCl₂.2H₂O). Other suitable metallic salts include cuprous bromide, cuprouscyanide, cuprous ferricyanide, zinc chloride, zinc bromide, zinc iodide,zinc cyanide, zinc ferrocyanide, zinc acetate, zinc sulfide, silverchloride, ferrous chloride, ferric chloride, ferrous ferricyanide,ferrous chloroplatinate, ferrous fluoride, ferrous sulfate, cobaltouschloride, cobaltic sulfate, cobaltous cyanide, nickel chloride, nickelcyanide, nickel sulfate, nickel carbonate, stannic chloride, stannouschloride hydrate, a complex of triphenylphosphine oxide and mixturesthereof.

Metals can also enhance the reaction rate, the preferred metals arecopper, iron zinc, and nickel due to their availability, handling, anddesired reactivity, as well as the enhanced thermal stability of theresulting polymer. Examples of other metals which may be used arechromium, molybdenum, vanadium, beryllium, silver, mercury, tin, lead,preferred metals are copper, iron zinc, and nickel due to theiravailability, handling, and desired reactivity, as well as the enhancedthermal stability of the resulting polymer. Examples of other metalswhich may be used are chromium, molybdenum, vanadium, beryllium, silver,mercury, tin, lead, antimony, calcium, barium, manganese, cobalt,palladium, and platinum Additional examples of metals and salts arefound in Mosher, Frank H. and Thomas, Arthur L., "PhthalocyanineCompounds", N.Y. Reinhold, 1963, p. 104-141.

The amine curing agent can be added in amounts up to about thestoichiometric amount. Preferably, the amount of the amine is from 1 to2 weight percent. The preferred amines are aromatic armines because oftheir superior thermal stability. The most preferred amines are the onesdisclosed and claimed in the previously cited co-pending patentapplication.

Without a postcure, the polymer exhibits low electrical conductiveproperties. As the polymer is postcured at a temperature from about 300°to 600° C. and preferably from 300° C. to 500° C., polymerizationcontinues, resulting in an unordered amorphous system. Electricalconductivity increases and is due to a variable range electron-hoppingmechanism. The non-planarity of the polymer at this point does notpermit the overlap of the p-orbitals within the polymers backbone, whichis essential for a polymer material to exhibit conductive properties. Ifthe polymer is held at a temperature above about 400° C., the polymerbegins to lose weight, indicating that the polymer is beginning topyrolyze.

During pyrolysis the temperature is raised above 400° C. but preferablebelow 2000° C. and most preferably from 500° C. and 1000° C., thepolymer gradually loses weight in a linear fashion and a blackcarbonaceous residue is produced. The atmosphere is inert and preferablyargon, nitrogen or helium. This is in contrast to other reportedpyrolytic systems, which lose most of their weight between 300° C. and600° C. In the early stages of pyrolysis (300°-500° C.), some homolyticfission of bonds probably occurs in the polymer accompanied by the lossof low molecular weight fragments and limited polycondensation of thearomatic rings. As the pyrolytic temperature is increased, a continuousnetwork of polyconjugated fused rings can develop within the amorphouscarbon matrix from carbonization between the ring systems. Moreover, asthe polycondensed ring structure increases in size, the overallresonance energy of the system is reduced PG,16 from enhanced p-orbitaldelocalization and sufficient p-orbital overlap will be developedresulting in the appearance of electrical conduction in the pyrolyzedpolymer.

The preferred pyrolysis proceeds in the following manner. The polymer isheated to a temperature from about 25° C. to about 300° C., at a rate upto about 20° C./min. After about 300° C. the temperature is increased ata rate preferably from about 0.2° C./min to about 1° C./min and mostpreferably from 0.3° C./min to 0.6° C./min, in an inert atmosphere, andpreferably in argon, helium or nitrogen. The preferred cooling rate isfrom 0.2° to 1° C./min and most preferred is from 0.3° to 0.6° C./min.The controlled heating and cooling rates minimize heat stresses in thepyrolyzed polymer. The pyrolytic temperature is maintained until thedesired conductivity is obtained.

The invention having been generally described, the following examples ofthe preparation and polymerization of4,4'-bis(3,4-dicyanothiophenoxy)biphenyl and the conversion into ahighly conductive organic polymer are given as particular embodiments ofthe invention and to demonstrate the practice and advantages thereof. Itis understood that the examples are given by way of illustrations andare not intended to limit the specification or the claims to follow inany manner.

EXAMPLES 1 Synthesis of 4,4'-Bis(3,4-Dicyanothiophenoxy)Biphenyl

A mixture containing 4, 4'-biphenyldithiol (4.29 g, 19.7 mmol),4-nitrophtholonitrile (6.81 g, 39.4 mmol), and anhydrous powderedpotassium carbonate (8.15 g, 59.1 mmol) in 40 ml of dry dimethylsulphoxide was stirred for 5 hours at 50° C. under a nitrogenatmosphere. After cooling, the reaction mixture was slowly poured into250 ml of dilute hydrochloric acid. The precipitate was isolated bysuction filtration, washed with water until the washings were clear andneutral, dried and then washed with hot absolute ethanol to yield 8.78 g(95%) of the desired product, m.p. 262°-264° C.

EXAMPLE 2 Polymerization of 4,4'-Bis(3,4 Dicyanothiophenoxy)

Biphenyl with Methylenedianiline

4,4'-Bis(3,4-dicyanothiophenoxy)biphenyl was melted and degassed atreduced pressure over a period of 2 hours. To the dark melt at 260° C.was added 4,4'-methylenedianiline (MDA, 1% by weight). The resultingmixture was cured by heating at 260° C. for 8 hours, at 280° C. for 4hours, and at 300° C. for 24 hours. Polymeric samples prepared in thismanner were characterized and pyrolyzed under an inert atmosphere atelevated temperatures.

EXAMPLE 3 Polymerization of 4,4'-Bis(3,4-Dicyanothiophenoxy)Biphenylwith 1,3-Bis(3-aminophenoxy)benzene

4,4'-Bis(3,4-dicyanothiophenoxy)biphenyl was melted and degassed atreduced pressure over a period of 4 hours. To the dark melt at 260° C.was added 1,3-bis(3-aminophenoxy)benzene (APB, 2% by weight) withstirring. The resulting mixture was cured by heating at 260° C. for 8hours, at 280° C. for 4 hours, and at 300° C. for 24 hours. Polymericsamples prepared in this manner were characterized and pyrolyzed underan inert atmosphere at elevated temperatures.

EXAMPLE 4 Pyrolysis of 4,4'-Bis(3,4-Dicyanothiophenoxy)Biphenyl-MDACured Polymer at 500° C.

The polymer (1.5 g) cured by using 1% by weight of MDA was heated undera oxygen-free argon atmosphere to 500° C. at a rate of 0.4° C./min. Thepolymer was annealed at 500° C. for 24 hours and then cooled back toroom temperature at 0.4° C./min. The polymer lost 13.7% weight duringthe heat treatment. The resulting pyrolysate exhibited a roomtemperature conductivity of 1.0×10⁻¹⁰ S/cm.

EXAMPLE 5 Pyrolysis of 4,4'-Bis(3,4-dicyanothiophenoxy)Biphenyl-APBCured Polymer at 600° C.

The polymer cured by using 2% by weight of APB was heated under anoxygen-free argon atmosphere to 600° C. at a rate of 0.4° C./min. Thepolymer was then annealed at 600° C. for 24 hours followed by coolingback to room temperature at 0.4° C./min. The polymeric composition lost10.9% weight during the heat treatment. The resulting pyrolysatedisplayed a room temperature conductivity of 8.3×10⁻⁴ S/cm.

EXAMPLE 6 Pyrolysis of 4,4'-Bis(3,4-Dicyanothiophenoxy)Biphenyl-APBCured Polymer at 500° C.

The polymer, cured by using 2% by weight of APB, was heated under anoxygen-free argon atmosphere to 500° C. at a rate of 0.4° C./min. Thepolymer was then annealed at 600° C. for 24 hours, followed by coolingback to room temperature at 0.4° C./min. The polymeric composition lost7.3% weight during the heat treatment. The resulting pyrolysate showed aroom temperature conductivity of 6.5×10⁻⁹ S/cm.

EXAMPLE 7 Pyrolysis of 4,4'-Bis(3,4-dicyanotrhiophenoxy)Biphenyl-MDACured Polymer at 700° C.

The polymer, cured by using 1% by weight of MDA, was heated under anoxygen-free argon atmosphere to 700° C. at a rate of 0.4° C./min. Thepolymer was then annealed at 700° C. for 24 hours, followed by coolingback to room temperature at 0.4° C./min. The polymeric composition lost20% weight during the heat treatment. The resulting pyrolysate exhibiteda room temperature conductivity of 8.4 S/cm.

EXAMPLE 8 Pyrolysis of 4,4'-Bis(3,4-Dicyanothiophenoxy)Biphenyl-MDACured Polymer at 900° C.

The polymer, cured by using 1% by weight of MDA, was heated under anoxygen-free argon atmosphere to 900° C. at a rate of 0.4° C./min. Thepolymer was annealed at 900° C. for 24 hours and then cooled back toroom temperature at 0.4° C./min. The polymeric composition lost 27.3%weight during the heat treatment. The resulting pyrolysate showed a roomtemperature conductivity of 6.1×10⁺¹ s/cm.

The polymers of the present invention can be processed to a definiteshaped component or film and pyrolyzed to a black conductive material.The maximum electrical conductivity is in the ranges approaching metalsand is achieved in a controlled and reproduceable manner in the absenceof an external chemical dopant. The pyrolysed polymer exhibitedexcellent thermal properties in air. The electrical conductivity isextremely stable in air.

An important aspect of the present invention is that the electricalbehavior of the pyrolysed polymer can be systematically changed from aninsulator to a semiconductor and made to approach metallic regions bycontrolling the thermal treatment process. For pyrolysis temperatures upto 700° C., the conductive behavior rapidly changed from an insulator toa semiconductor. Between 700° C. and 900° C., a less dramatic increasein conductivity is observed with increasing pyrolytic temperatures. Forexample, when annealed at 600°, 700° and 900° C. for 24 h, the pyrolyzedpolymer exhibits a conductivity of 8.3×10⁻⁴, 8.4 and 61 S cm⁻¹,respectively. The conductivity increase appears to confirm the stepwisethermal decomposition discussed previously and the formation of planarpolycondensed rings at elevated temperatures. An enhancement in the sizeof the conductive fused ring components allows current to flow longerdistances in the individual conductive species. At the same time, thetransport of current by interparticle contact (tunnelling or hopping) isreduced due to a reduction in the number of fused rings and theemergence of larger polycondensed ring systems.

Additional information and discussion on the subject monomers andpolymers is available in Keller, Teddy M. and Gratz, Roy F. HighTemperature Intrinsically Conductive Polymer in Polymer Communication,vol. 28, pp. 334 to 336 (Dec. '87).

Obviously many modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:
 1. A polymer of a phthalonitrile monomer representedby the formula: ##STR4## wherein A is selected from the group consistingof an aliphatic hydrocarbon having from 2 to 30 carbon atoms, a cyclichydrocarbon having from 4 to 30 carbon atoms, and mixtures thereof.
 2. Apolymer as described in claim 1 wherein said cyclic hydrocarbon hasmultiple aromatic rings.
 3. A polymer as described in claim 2 whereinsaid aromatic rings are fuses.
 4. A polymer as described in claim 2wherein said cyclic hydrocarbon is biphenyl.
 5. A polymer as describedin claim 2 wherein said cyclic hydrocarbon is selected from the groupconsisting of naphthalene, anthracene, and mixtures thereof.
 6. A methodcomprising polymerizing said monomer of claim 1 to a polymer whichcomprises heating said monomer at a temperature above the melting pointthereof but below the decomposition temperature of said monomer or saidpolymer.